Semiconductor device and control method of the same

ABSTRACT

Increases of circuit scale and power consumption are suppressed while frequency deviation is kept within a predetermined allowable range. A semiconductor device according to an embodiment includes a variable load capacity circuit including a plurality of load capacity elements coupled in parallel to one end of a crystal resonator and a plurality of switches that are respectively serially coupled to the load capacity elements, and a switch control unit that controls ON/OFF of the switches on the basis of information to be an index of frequency deviation due to temperature change of a frequency signal obtained by oscillating the crystal resonator. The switch control unit changes the number of switches that will be turned ON among the plurality of switches so that an absolute value of the frequency deviation becomes small when the information is not included in a predetermined allowable range.

CROSS-REFERENCE TO RELATED APPLICATIONS

The disclosure of Japanese Patent Application No. 2017-114988 filed onJun. 12, 2017 including the specification, drawings and abstract isincorporated herein by reference in its entirety.

BACKGROUND

The present invention relates to a semiconductor device and a controlmethod of the same.

A crystal oscillator is widely used to generate a reference frequencysignal in, for example, a semiconductor device for wirelesscommunication. The crystal oscillator includes a crystal resonator andan oscillation circuit that oscillates the crystal resonator.

An oscillation frequency of the crystal resonator has a slighttemperature dependence, and an oscillation frequency of a normal AT-cutcrystal resonator varies in a cubic curve with respect to temperature.As an example, in a temperature range of −40 to 85° C., the oscillationfrequency has a frequency deviation of about ±20 to 30 ppm.

Japanese Unexamined Patent Application Publication Nos. 2008-300978 and2013-098860 disclose a temperature compensated crystal oscillator (TCXO)including a temperature compensation circuit that performs control so asto cancel the temperature dependence in a cubic curve described aboveand maintain the oscillation frequency constant. The temperaturecompensated crystal oscillator can reduce the frequency deviation to,for example, ±1 ppm or less.

SUMMARY

In the temperature compensated crystal oscillator as disclosed inJapanese Unexamined Patent Application Publication Nos. 2008-300978 and2013-098860, as described above, the temperature dependence in a cubiccurve is cancelled and the frequency deviation is reduced to, forexample, ±1 ppm or less. Therefore, it is necessary to store a hugeamount of data and continuously change load capacity by using an analogvariable capacitance element such as a varicap. Therefore, there is aproblem that circuit scale and power consumption increase.

Other objects and novel features will become apparent from thedescription of the present specification and the accompanying drawings.

A semiconductor device according to an embodiment includes a variableload capacity circuit including a plurality of load capacity elementscoupled in parallel to one end of a crystal resonator and a plurality ofswitches that are respectively serially coupled to the load capacityelements, and a switch control unit that controls ON/OFF of the switcheson the basis of information to be an index of frequency deviation due totemperature change of a frequency signal obtained by oscillating thecrystal resonator. The switch control unit changes the number ofswitches that will be turned ON among the plurality of switches so thatan absolute value of the frequency deviation becomes small when theinformation is not included in a predetermined allowable range.

According to the embodiment, it is possible to suppress increases ofcircuit scale and power consumption while keeping the frequencydeviation within a predetermined allowable range.

BRIEF DESCRIPTION OF THE DRAWINGS

FIG. 1 is a block diagram showing a configuration of a semiconductordevice 100 according to a first embodiment.

FIG. 2 is a detailed block diagram showing the configuration of thesemiconductor device 100 according to the first embodiment.

FIG. 3 is a diagram showing an example of a table stored in a memoryunit MEM in the semiconductor device 100 according to the firstembodiment.

FIG. 4 is a block diagram showing an example of a configuration of aradio transceiver circuit RFT included in the semiconductor device 100according to the first embodiment.

FIG. 5 is a graph showing an example of temperature dependence offrequency deviation Δf/fn of a crystal resonator CU.

FIG. 6 is a graph showing an example of load capacity dependence of thefrequency deviation Δf/fn of the crystal resonator CU.

FIG. 7 is a flowchart showing a control method of the semiconductordevice according to the first embodiment.

FIG. 8 is a detailed block diagram showing a configuration of asemiconductor device 200 according to a second embodiment.

FIG. 9 is a block diagram showing an example of a configuration of aradio transceiver circuit RFT included in the semiconductor device 200according to the second embodiment.

FIG. 10 is a diagram showing an example of a table stored in a memoryunit MEM in the semiconductor device 200 according to the secondembodiment.

FIG. 11 is a flowchart showing a control method of the semiconductordevice according to the second embodiment.

DETAILED DESCRIPTION

For clarity of explanation, the following description and drawings areappropriately omitted and simplified. The components shown in thedrawings as functional blocks that perform various processing can beformed by a CPU, a memory, and other circuits as hardware and arerealized by a program and the like loaded in a memory as software.Therefore, it should be understood by those skilled in the art that thefunctional blocks can be realized in various forms by only hardware,only software, or a combination of these, and the functional blocks arenot limited to any one of hardware, software, and a combination ofthese. In the drawings, the same components are denoted by the samereference symbols and redundant description is omitted as appropriate.

First Embodiment

<Configuration of Semiconductor Device 100>

First, a semiconductor device according to a first embodiment will bedescribed with reference to FIG. 1. FIG. 1 is a block diagram showing aconfiguration of a semiconductor device 100 according to the firstembodiment. As shown in FIG. 1, the semiconductor device 100 accordingto the first embodiment includes a crystal resonator CU, an oscillationcircuit OC, a variable load capacity circuit CL, and a switch controlunit SC. Here, the variable load capacity circuit CL includes 2n (n is anatural number greater than or equal to 2) load capacity elements C1 ato Cna and C1 b to Cnb and 2n switches S1 a to Sna and S1 b to Snb.

As shown in FIG. 1, in the semiconductor device 100, the crystalresonator CU is oscillated by the oscillation circuit OC and a frequencysignal fs is outputted from the oscillation circuit OC. One end of thecrystal resonator CU is coupled with one ends of all the load capacityelements C1 a to Cna included in the variable load capacity circuit CL.The other ends of the load capacity elements C1 a to Cna arerespectively coupled with one ends of the switches S1 a to Sna includedin the variable load capacity circuit CL. The other ends of all theswitches S1 a to Sna are coupled to ground.

That is, the load capacity elements C1 a to Cna are respectively andserially coupled with the switches S1 a to Sna between one end of thecrystal resonator CU and the ground. In other words, the seriallycoupled n pairs of the load capacity elements C1 a to Cna and theswitches S1 a to Sna are coupled in parallel between one end of thecrystal resonator CU and the ground. The switches S1 a to Sna are, forexample, MOS (Metal-Oxide-Semiconductor) transistors.

The switches S1 a to Sna may be coupled to one end of the crystalresonator CU, and the load capacity elements C1 a to Cna may be coupledto the ground. The capacities of each of the load capacity elements C1 ato Cna need not be equal. However, when the capacities are equal, it iseasy to control and manufacture the load capacity element C1 a to Cna.

Similarly, the other end of the crystal resonator CU is coupled with oneends of all the load capacity elements C1 b to Cnb included in thevariable load capacity circuit CL. The other ends of the load capacityelements C1 b to Cnb are respectively coupled with one ends of theswitches S1 b to Snb included in the variable load capacity circuit CL.The other ends of all the switches S1 b to Snb are coupled to theground.

That is, the load capacity elements C1 b to Cnb are respectively andserially coupled with the switches S1 b to Snb between one end of thecrystal resonator CU and the ground. In other words, the seriallycoupled n pairs of the load capacity elements C1 b to Cnb and theswitches S1 b to Snb are coupled in parallel between one end of thecrystal resonator CU and the ground. The switches S1 b to Snb are, forexample, MOS transistors.

The switches S1 b to Snb may be coupled to the other end of the crystalresonator CU, and the load capacity elements C1 b to Cnb may be coupledto the ground. The capacities of each of the load capacity elements C1 bto Cnb need not be equal. However, when the capacities are equal, it iseasy to control and manufacture the load capacity element C1 b to Cnb.

The switch control unit SC acquires information to be an index offrequency deviation of the frequency signal fs due to temperaturechange. The switch control unit SC controls ON/OFF of n pairs of theswitches S1 a to Sna and S1 b to Snb through a bus BUS including nsignal lines on the basis of the acquired information. Here, ON/OFF ofthe switches S1 a and Sib, which are a pair, is controlled by the samesignal. Similarly, ON/OFF of the switches S2 a and S2 b to switches Snaand Snb, which are pairs, is controlled by the same signal,respectively.

When the acquired information is not included in a predeterminedallowable range, the switch control unit SC changes the number ofswitches that will be turned ON among the switches S1 a to Sna and S1 bto Snb so that an absolute value of the frequency deviation of thefrequency signal fs outputted from the oscillation circuit OC becomessmall. Thereby, the number of the load capacity elements C1 a to Cnathat are coupled between one end of the crystal resonator CU and theground is changed, and the load capacity coupled to the one end of thecrystal resonator CU is changed. Similarly, the number of the loadcapacity elements C1 b to Cnb that are coupled between the other end ofthe crystal resonator CU and the ground is changed, and the loadcapacity coupled to the other end of the crystal resonator CU ischanged.

In the semiconductor device 100 according to the present embodiment,both the load capacities coupled to both ends of the crystal resonatorCU can be changed. However, it is all right that the load capacitycoupled to at least one end of the crystal resonator CU can be changed.

<Description of Effects>

As described above, the semiconductor device 100 according to thepresent embodiment includes the switch control unit SC that controlsON/OFF of the switches S1 a to Sna and S1 b to Snb on the basis of theinformation to be an index of frequency deviation of the frequencysignal fs due to temperature change. Specifically, when the acquiredinformation is not included in the predetermined allowable range, theswitch control unit SC changes the number of switches that will beturned ON among the switches S1 a to Sna and S1 b to Snb so that theabsolute value of the frequency deviation of the frequency signal fsbecomes small. Therefore, it is possible to change the load capacitiescoupled to both ends of the crystal resonator CU and keep the frequencydeviation of the frequency signal fs outputted from the oscillationcircuit OC within a predetermined allowable range.

On the other hand, in the semiconductor device 100 according to thepresent embodiment, it is not necessary to store a huge amount of datain order to cancel a cubic curve shaped temperature dependence as in atemperature compensated crystal oscillator (TCXO). Further, it is notnecessary to continuously change the load capacity, so that an analogvariable capacitance element such as a varicap is not required.Therefore, in the semiconductor device 100 according to the presentembodiment, it is possible to suppress increases of circuit scale andpower consumption as compared with in the temperature compensatedcrystal oscillator.

As described above, in the semiconductor device 100 according to thepresent embodiment, it is possible to suppress increases of circuitscale and power consumption while keeping the frequency deviation withina predetermined allowable range.

<Detailed Configuration of Semiconductor Device 100>

Next, the semiconductor device according to the first embodiment will bedescribed in more detail with reference to FIG. 2. FIG. 2 is a detailedblock diagram showing the configuration of the semiconductor device 100according to the first embodiment. As shown in FIG. 2, the semiconductordevice 100 according to the first embodiment includes a temperaturesensor TS, a memory unit MEM, a radio transceiver circuit RFT, and anantenna AN in addition to the crystal resonator CU, the oscillationcircuit OC, the variable load capacity circuit CL, and the switchcontrol unit SC shown in FIG. 1.

As shown in FIG. 2, the oscillation circuit OC, the variable loadcapacity circuit CL, the switch control unit SC, the memory unit MEM,the temperature sensor TS, and the radio transceiver circuit RFT areintegrated in a radio transceiver LSI 10 composed of a semiconductorchip. The crystal resonator CU and the antenna AN are externallyattached to terminals provided to the radio transceiver LSI 10.

As shown in FIG. 2, the oscillation circuit OC includes an invertor INV,a limiting resistor Rd, and a feedback resistor Rf. The oscillationcircuit OC oscillates the crystal resonator CU and outputs the frequencysignal fs.

The input of the invertor INV is coupled to one end of the crystalresonator CU, and the frequency signal fs is outputted from the outputof the inverter INV.

The limiting resistor Rd is provided between the output of the invertorINV and the other end of the crystal resonator CU. The limiting resistorRd prevents an excessive current from flowing into the oscillationcircuit OC.

The feedback resistor Rf is provided in parallel with the invertor INV.Specifically, one end of the feedback resistor Rf is coupled to theinput of the invertor INV and the other end of the feedback resistor Rfis coupled to the output of the invertor INV. The feedback resistor Rffeeds back a current from the output of the invertor INV to the input ofthe invertor INV and causes oscillation to continue.

One end of the crystal resonator CU is coupled with one ends of all theload capacity elements C1 a to Cna included in the variable loadcapacity circuit CL. The other ends of the load capacity elements C1 ato Cna are respectively coupled with one ends of the switches S1 a toSna included in the variable load capacity circuit CL. The other ends ofall the switches S1 a to Sna are coupled to the ground.

That is, the load capacity elements C1 a to Cna are respectively andserially coupled with the switches S1 a to Sna between one end of thecrystal resonator CU and the ground. In other words, the seriallycoupled n pairs of the load capacity elements C1 a to Cna and theswitches S1 a to Sna are coupled in parallel between one end of thecrystal resonator CU and the ground. The switches S1 a to Sna are, forexample, MOS (Metal-Oxide-Semiconductor) transistors.

Similarly, the other end of the crystal resonator CU is coupled with oneends of all the load capacity elements C1 b to Cnb included in thevariable load capacity circuit CL. The other ends of the load capacityelements C1 b to Cnb are respectively coupled with one ends of theswitches S1 b to Snb included in the variable load capacity circuit CL.The other ends of all the switches S1 b to Snb are coupled to theground.

That is, the load capacity elements C1 b to Cnb are respectively andserially coupled with the switches S1 b to Snb between one end of thecrystal resonator CU and the ground. In other words, the seriallycoupled n pairs of the load capacity elements C1 b to Cnb and theswitches S1 b to Snb are coupled in parallel between one end of thecrystal resonator CU and the ground. The switches S1 b to Snb are, forexample, MOS transistors.

The switch control unit SC acquires temperature information from thetemperature sensor TS. The switch control unit SC controls ON/OFF of then pairs of switches S1 a to Sna and S1 b to Snb through the bus BUSincluding n signal lines on the basis of the acquired temperatureinformation. Here, ON/OFF of the switches S1 a and Sib, which are apair, is controlled by the same signal. Similarly, ON/OFF of theswitches S1 a and S1 b to the switches Sna and Snb, which are pairs, iscontrolled by the same signal, respectively.

Further, when the switch control unit SC acquires temperatureinformation from the temperature sensor TS, the switch control unit SCrefers to a table stored in the memory unit MEM and controls ON/OFF ofthe switches S1 a to Sna and S1 b to Snb. Therefore, it is possible toquickly process ON/OFF control of the switches S1 a to Sna and S1 b toSnb.

As described later in detail with reference to FIG. 3, the table shows acorrespondence relationship between a temperature range and the numberof pairs of switches to be ON in the temperature range for each type(crystal type) of the crystal resonator CU. This is because when thecrystal type is different, the frequency characteristic is somewhatdifferent. Therefore, the switch control unit SC acquires a crystal typesignal ct that indicates a type of the crystal resonator CU to be used.

When the acquired temperature information is not included in apredetermined allowable range, the switch control unit SC changes thenumber of switches that will be turned ON among the switches S1 a to Snaand S1 b to Snb so that the absolute value of the frequency deviation ofthe frequency signal fs outputted from the oscillation circuit OCbecomes small. Thereby, the number of the load capacity elements C1 a toCna that are coupled between one end of the crystal resonator CU and theground is changed, and the load capacity coupled to the one end of thecrystal resonator CU is changed. Similarly, the number of the loadcapacity elements C1 b to Cnb that are coupled between the other end ofthe crystal resonator CU and the ground is changed, and the loadcapacity coupled to the other end of the crystal resonator CU ischanged.

The temperature sensor TS indirectly detects a temperature of thecrystal resonator CU. A bandgap reference circuit that generates areference voltage for an analog circuit that includes the radiotransceiver circuit RFT may be used as the temperature sensor TS. Inthis case, it is not necessary to separately provide the temperaturesensor TS, so that it is possible to suppress increase in the circuitscale.

The memory unit MEM stores a table that is referred to by the switchcontrol unit SC. Here, FIG. 3 is a diagram showing an example of thetable stored in the memory unit MEM in the semiconductor device 100according to the first embodiment. As shown in FIG. 3, the table showsthe type (crystal type) of the crystal resonator CU, the temperaturerange, and the number of pairs of switches to be ON. Specifically, thetable shows a correspondence relationship between the temperature rangeand the number of pairs of switches to be ON for each of the differentcrystal types A and B. When the crystal type is different, the frequencycharacteristics such as the temperature dependence shown in FIG. 5described later and the load capacity dependence shown in FIG. 6described later are different at least to a certain extent. Therefore,the table shows a correspondence relationship between the temperaturerange and the number of pairs of switches to be ON for each crystaltype.

As shown in FIG. 3, in both of the crystal types A and B, 10 pairs ofswitches are turned ON in an allowable temperature range of the crystalresonator CU (−40 to 85° C. in the example of FIG. 3). In both of thecrystal types A and B, as the temperature rises exceeding the allowabletemperature range, the number of pairs of switches to be ON isincreased.

In the example of FIG. 3, specifically, in the case of the crystal typeA, 11 pairs of switches are turned ON in a temperature range of 85 to95° C., 12 pairs of switches are turned ON in a temperature range of 95to 100° C., and 13 pairs of switches are turned ON in a temperaturerange of 100 to 105° C. On the other hand, in the case of the crystaltype B, 11 pairs of switches are turned ON in a temperature range of 85to 100° C. and 12 pairs of switches are turned ON in a temperature rangeof 100 to 105° C. As a matter of course, the temperature ranges and thenumbers of pairs of switches to be ON shown in FIG. 3 are only anexample and can be appropriately changed.

The radio transceiver circuit RFT generates a transmission RF (RadioFrequency) signal from transmission data td received from outside byusing the frequency signal fs outputted from the oscillation circuit OCand wirelessly transmits the transmission RF signal through the antennaAN. On the other hand, the radio transceiver circuit RFT wirelesslyreceives a reception RF (Radio Frequency) signal through the antenna AN,generates a reception data rd from the reception RF signal, andtransmits the reception data rd to the outside.

FIG. 4 is a block diagram showing an example of a configuration of theradio transceiver circuit RFT included in the semiconductor device 100according to the first embodiment. The superheterodyne type radiotransceiver circuit RFT includes a baseband processing unit BBP, an IQmodulator IQM, IF (Intermediate Frequency) amplifiers IFA1 and IFA2, PLL(Phase Locked Loop) circuits PLL1 and PLL2, a frequency synthesizer FS,an up-converter UC, a power amplifier PA, an output filter OF, alow-noise amplifier LNA, an RF (Radio Frequency) mixer RFM, an IF(Intermediate Frequency) mixer IFM. Various filters are omitted.

The baseband processing unit BBP encodes the transmission data tdreceived from outside into a transmission IQ signal and transmits thetransmission IQ signal to the IQ modulator IQM. On the other hand, thebaseband processing unit BBP decodes a reception IQ signal received fromthe IF mixer IFM into reception data rd and transmits the reception datard to the outside.

Hereinafter, a flow of the transmission data td will be described.

In the IQ modulator IQM, an IF (Intermediate Frequency) signal outputtedfrom the PLL circuit PLL1 is orthogonally modulated by the transmissionIQ signal outputted from the baseband processing unit BBP and atransmission IF signal is generated. Here, the IF signal outputted fromthe PLL circuit PLL1 is generated from the frequency signal fs outputtedfrom the oscillation circuit OC.

The transmission IF signal outputted from the IQ modulator IQM isamplified by the IF amplifier IFA1. The amplified transmission IF signalis mixed with a frequency signal outputted from the frequencysynthesizer FS and up-converted to a transmission RF signal in theup-converter UC. Here, the frequency signal outputted from the frequencysynthesizer FS is generated from the frequency signal fs outputted fromthe oscillation circuit OC.

The transmission RF signal outputted from the up-converter UC isamplified by the power amplifier PA and thereafter wirelesslytransmitted from the antenna AN through the output filter OF. Here, theoutput filter OF suppresses propagation of the transmission RF signal,which is wirelessly transmitted from the antenna AN, to the low-noiseamplifier LNA.

Next, a flow of the reception data rd will be described.

The reception RF signal wirelessly received by the antenna AN isinputted into the low-noise amplifier LNA through the output filter OFand amplified by the low-noise amplifier LNA. Here, the output filter OFsuppresses propagation of the reception RF signal, which is wirelesslyreceived by the antenna AN, to the power amplifier PA.

The amplified reception RF signal is mixed with the aforementionedfrequency signal outputted from the frequency synthesizer FS anddown-converted to a reception IF signal in the RF mixer RFM. Thereception IF signal outputted from the RF mixer RFM is amplified by theIF amplifier IFA2.

The amplified reception IF signal is mixed with an IF signal outputtedfrom the PLL circuit PLL2 in the IF mixer IFM and demodulated into areception IQ signal. Then, the reception IQ signal outputted from the IFmixer IFM is decoded into the reception data rd by the basebandprocessing unit BBP.

<Description of Effects>

As described above, the semiconductor device 100 according to thepresent embodiment includes the switch control unit SC that controlsON/OFF of the switches S1 a to Sna and S1 b to Snb on the basis of theacquired temperature information. Specifically, when the acquiredtemperature information is not included in a predetermined allowablerange, the switch control unit SC changes the number of switches thatwill be turned ON among the switches S1 a to Sna and S1 b to Snb so thatthe absolute value of the frequency deviation of the frequency signal fsoutputted from the oscillation circuit OC becomes small. Therefore, itis possible to change the load capacities coupled to both ends of thecrystal resonator CU and keep the frequency deviation of the frequencysignal fs outputted from the oscillation circuit OC within apredetermined allowable range.

Here, FIG. 5 is a graph showing an example of temperature dependence offrequency deviation Δf/fn of the crystal resonator CU. The horizontalaxis indicates the temperature [° C.] and the vertical axis indicatesthe frequency deviation Δf/fn [ppm]. Here, Δf is a difference between anoscillation frequency f and a nominal frequency fn of the crystalresonator CU at each temperature, that is, a frequency shift due totemperature. The frequency shift due to temperature is Δf=f−fn.

As shown in FIG. 5, the frequency deviation Δf/fn varies in a cubiccurve with respect to temperature. As shown in FIG. 5, in a compensationtemperature range, that is, the allowable temperature range of thecrystal resonator CU (−40 to 85° C. in the example of FIG. 5), thefrequency deviation Δf/fn is within a predetermined allowable range (±30ppm in the example of FIG. 5).

On the other hand, if a use temperature range is expanded to, forexample, −40 to 105° C., the frequency deviation Δf/fn of the crystalresonator CU exceeds an upper limit (30 ppm in the example of FIG. 5) ofan allowable deviation.

Here, the allowable range of the frequency deviation Δf/fn isappropriately set by a communication standard or the like. For example,the allowable range of the frequency deviation in a short-range radiocommunication standard such as Bluetooth (registered trademark) LowEnergy is ±50 ppm. Therefore, it is considered that the allowable rangeof the frequency deviation is set to, for example, about ±30 ppmconsidering a margin with respect to the standard. Such an allowablerange is one digit larger than that of a communication standard formobile phone (the allowable range of the frequency deviation is ±3 ppm),so that a frequency deviation as small as that of the temperaturecompensated crystal oscillator (TCXO) (for example, ±1 ppm or less) isnot required.

Here, FIG. 6 is a graph showing an example of load capacity dependenceof the frequency deviation Δf/fn of the crystal resonator CU. Thehorizontal axis indicates the load capacity [pF] and the vertical axisindicates the frequency deviation Δf/fn [ppm]. In the example of FIG. 6,a load capacity of 12.5 pF is coupled to the crystal resonator CU sothat the frequency deviation Δf/fn becomes 0 at 25° C. Here, as shown inFIG. 6, when the load capacity coupled to the crystal resonator CUincreases, the frequency deviation Δf/fn monotonically decreases.

Therefore, in the semiconductor device 100 according to the presentembodiment, when an acquired temperature exceeds, for example, anallowable temperature upper limit (85° C. in the example of FIG. 5), thenumber of switches that will be turned ON among the switches S1 a to Snaand S1 b to Snb is increased. That is to say, as shown in FIG. 6, it ispossible to decrease the frequency deviation Δf/fn of the frequencysignal fs outputted from the oscillation circuit OC by increasing theload capacities coupled to both ends of the crystal resonator CU. As aresult, it is possible to keep the frequency deviation Δf/fn of thefrequency signal fs within a range of allowable deviation (30 ppm in theexample of FIG. 5).

On the other hand, in the semiconductor device 100 according to thepresent embodiment, it is not necessary to store a huge amount of datain order to cancel a cubic curve shaped temperature dependence as in thetemperature compensated crystal oscillator (TCXO). Further, it is notnecessary to continuously change the load capacity, so that an analogvariable capacitance element such as a varicap is not required.Therefore, in the semiconductor device 100 according to the presentembodiment, it is possible to suppress increases of circuit scale andpower consumption as compared with in the temperature compensatedcrystal oscillator.

As described above, in the semiconductor device 100 according to thepresent embodiment, it is possible to suppress increases of circuitscale and power consumption while keeping the frequency deviation withina predetermined allowable range.

<Control Method of Semiconductor Device>

Next, a control method of the semiconductor device according to thefirst embodiment will be described with reference to FIG. 7. FIG. 7 is aflowchart showing the control method of the semiconductor deviceaccording to the first embodiment. In the description of FIG. 7, FIG. 1is appropriately referred to.

As shown in FIG. 7, first, the temperature sensor TS shown in FIG. 1detects the temperature of the crystal resonator CU (step ST11).

Next, the switch control unit SC determines whether or not thetemperature detected by the temperature sensor TS is within an in-usetemperature range in the table stored in the memory unit MEM (see FIG.3) (step ST12). A temperature range that is used first is the allowabletemperature range.

When the detected temperature is within the in-use temperature range inthe table shown in FIG. 3 (step ST12 YES), the switch control unit SCdoes not change the number of switches (the number of pairs of switches)to be ON and ends switch control without change. On the other hand, whenthe detected temperature is not within the in-use temperature range inthe table shown in FIG. 3 (step ST12 NO), the switch control unit SCchanges the number of switches (the number of pairs of switches) to beON to a number corresponding to a temperature range including thedetected temperature (step ST13) and then ends the switch control.

In the semiconductor device 100 according to the present embodiment, toreduce power consumption, the power source is turned off and theoscillation circuit OC stops every time packet transmission/reception iscompleted. Therefore, for example, every time a packet istransmitted/received, the switch control unit SC repeats the controlshown in FIG. 7 after the power source is turned on until packettransmission/reception is completed. For example, in the case ofBluetooth Low Energy, packet transmission/reception of 625 μs isrepeated at intervals of 7.5 ms. The control shown in FIG. 7 may berepeated every time a plurality of times of packettransmission/reception are performed, instead of every time a packet istransmitted/received. At a timing when the number of switches to be ONis changed, load capacity elements whose potentials are different arecoupled, so that the frequency of the frequency signal fs outputted fromthe oscillation circuit OC may largely vary. Therefore, it is preferablethat the timing when the number of switches to be ON is changed is otherthan during transmission/reception of a packet.

Second Embodiment

<Detailed Configuration of Semiconductor Device 200>

Next, a semiconductor device according to the second embodiment will bedescribed in detail with reference to FIG. 2. FIG. 8 is a detailed blockdiagram showing a configuration of the semiconductor device 200according to the second embodiment.

In the semiconductor device 100 according to the first embodiment shownin FIG. 2, the switch control unit SC controls ON/OFF of the n pairs ofswitches S1 a to Sna and S1 b to Snb on the basis of the temperatureinformation acquired from the temperature sensor TS. On the other hand,in the semiconductor device 200 according to the second embodiment shownin FIG. 8, the switch control unit SC controls ON/OFF of the n pairs ofswitches S1 a to Sna and S1 b to Snb on the basis of an automaticfrequency control (AFC) signal afc acquired from the radio transceivercircuit RFT. Therefore, the semiconductor device 200 according to thesecond embodiment does not require the temperature sensor TS.

The AFC signal afc is a difference between a frequency (hereinafterreferred to as a master frequency) of a reception RF signal receivedfrom a master wireless apparatus (not shown in the drawings) and thefrequency of the frequency signal fs outputted from the oscillationcircuit OC. The master frequency is maintained constant at all times andis equal to the nominal frequency fn of the crystal resonator CU. Thatis to say, the AFC signal afc corresponds to a frequency shift due totemperature of the frequency signal fs outputted from the oscillationcircuit OC.

Here, FIG. 9 is a block diagram showing an example of a configuration ofthe radio transceiver circuit RFT included in the semiconductor device200 according to the second embodiment. The radio transceiver circuitRFT shown in FIG. 9 has the same circuit configuration as that of theradio transceiver circuit RFT shown in FIG. 4. As shown in FIG. 9, thebaseband processing unit BBP includes a reception detection unit RDU.The reception detection unit RDU generates the AFC signal afc andoutputs the AFC signal afc to the switch control unit SC. In FIG. 4, thereception detection unit RDU is omitted.

As shown in FIG. 9, the reception detection unit RDU receives thefrequency of the reception IQ signal outputted from the IF mixer IFM andthe frequency of the frequency signal fs outputted from the oscillationcircuit OC. Here, the frequency of the reception IQ signal is the masterfrequency. Therefore, the reception detection unit RDU can generate theAFC signal afc corresponding to a difference between the masterfrequency and the frequency of the frequency signal fs outputted fromthe oscillation circuit OC.

As shown in FIG. 8, the switch control unit SC acquires the AFC signalafc, that is, the frequency shift of the frequency signal fs, from theradio transceiver circuit RFT. Then, the switch control unit SC controlsON/OFF of n pairs of the switches S1 a to Sna and S1 b to Snb throughthe bus BUS including n signal lines on the basis of the acquiredfrequency shift of the frequency signal fs. Here, ON/OFF of the switchesS1 a and Sib, which are a pair, is controlled by the same signal.Similarly, ON/OFF of the switches S1 a and S1 b to the switches Sna andSnb, which are pairs, is controlled by the same signal, respectively.

When the switch control unit SC acquires the frequency shift of thefrequency signal fs, the switch control unit SC refers to the tablestored in the memory unit MEM and controls ON/OFF of the switches S1 ato Sna and S1 b to Snb. Therefore, it is possible to quickly processON/OFF control of the switches S1 a to Sna and S1 b to Snb.

When the acquired frequency shift of the frequency signal fs is notincluded in a predetermined allowable range, the switch control unit SCchanges the number of switches that will be turned ON among the switchesS1 a to Sna and S1 b to Snb so that the absolute value of the frequencydeviation of the frequency signal fs outputted from the oscillationcircuit OC becomes small. Thereby, the number of the load capacityelements C1 a to Cna that are coupled between one end of the crystalresonator CU and the ground is changed, and the load capacity coupled tothe one end of the crystal resonator CU is changed. Similarly, thenumber of the load capacity elements C1 b to Cnb that are coupledbetween the other end of the crystal resonator CU and the ground ischanged, and the load capacity coupled to the other end of the crystalresonator CU is changed.

Here, FIG. 10 is a diagram showing an example of the table stored in thememory unit MEM in the semiconductor device 200 according to the secondembodiment. As shown in FIG. 10, the table shows the amount of frequencylowering when one pair of switches to be ON is added for each type(crystal type) of crystal resonator CU. In the example of FIG. 10, theamount of frequency lowering when one pair of switches to be ON is addedis shown for each of different crystal types A, B, and C. The amount offrequency lowering can be acquired from the load capacity dependence ofthe frequency deviation Δf/fn of the crystal resonator CU shown in FIG.6.

In the example of FIG. 10, specifically, in the case of crystal type A,when one pair of switches to be ON is added, the frequency changes by −8kHz. In the case of crystal type B, when one pair of switches to be ONis added, the frequency changes by −13 kHz. In the case of crystal typeC, when one pair of switches to be ON is added, the frequency changes by−10 kHz. As a matter of course, the values in FIG. 10 of frequencychange when one pair of switches to be ON is added are only an exampleand can be appropriately changed.

Specific examples will be described below.

When the nominal frequency fn is 2.4 GHz and the allowable deviation ofthe frequency deviation Δf/fn of the frequency signal fs is ±30 ppm, theallowable range of the frequency shift of the frequency signal fs is ±72kHz (=2.4 GHz×(±30 ppm)). Therefore, when the acquired AFC signal afc,that is, the frequency shift of the frequency signal fs, is included inthe allowable range (±72 kHz), the switch control unit SC does notchange the number of switches to be ON.

On the other hand, when the frequency shift of the acquired frequencysignal fs is not included in the allowable range (±72 kHz), the switchcontrol unit SC divides a value of the frequency shift by the amount offrequency lowering when one pair of switches to be ON is added shown inFIG. 10. When the obtained quotient is a positive value, the number ofpairs of switches to be ON is increased by a value corresponding to theintegral part of the quotient, and when obtained quotient is a negativevalue, the number of pairs of switches to be ON is decreased by a valuecorresponding to the integral part of the quotient.

As a specific example, it is assumed that the crystal type B is usedwhere the amount of frequency lowering when one pair of switches to beON is added shown in FIG. 10 is 13 kHz and the AFC signal afc, that is,a value of the frequency shift of the frequency signal fs, is notincluded in the allowable range (±72 kHz). When the value of thefrequency shift is 80 KHz that exceeds an allowable upper limit value 72kHz, six pairs of switches to be ON are added because 80 kHz/13 kHz=6.1. . . . On the other hand, when the value of the frequency shift is −80KHz that falls below an allowable lower limit value −72 kHz, six pairsof switches to be ON are subtracted because −80 kHz/13 kHz=−6.1 . . . .By such a control, it is possible to keep the frequency deviation of thefrequency signal fs outputted from the oscillation circuit OC within apredetermined allowable range.

<Description of Effects>

As described above, the semiconductor device 200 according to thepresent embodiment includes the switch control unit SC that controlsON/OFF of the switches S1 a to Sna and S1 b to Snb on the basis of theAFC signal afc indicating the shift of the frequency signal fs due totemperature change. Specifically, when the shift of the acquiredfrequency signal fs is not included in a predetermined allowable range,the switch control unit SC changes the number of switches that will beturned ON among the switches S1 a to Sna and S1 b to Snb so that theabsolute value of the frequency deviation of the frequency signal fsbecomes small. Therefore, it is possible to change the load capacitiescoupled to both ends of the crystal resonator CU and keep the frequencydeviation of the frequency signal fs outputted from the oscillationcircuit OC within a predetermined allowable range.

On the other hand, also in the semiconductor device 200 according to thepresent embodiment, it is not necessary to store a huge amount of datain order to cancel a cubic curve shaped temperature dependence as in thetemperature compensated crystal oscillator (TCXO). Further, it is notnecessary to continuously change the load capacity, so that an analogvariable capacitance element such as a varicap is not required.Therefore, also in the semiconductor device 200 according to the presentembodiment, it is possible to suppress increases of circuit scale andpower consumption as compared with in the temperature compensatedcrystal oscillator.

Further, in the semiconductor device 200 according to the presentembodiment, the temperature sensor TS is not required, so that it ispossible to further suppress increases of circuit scale and powerconsumption as compared with in the semiconductor device 100 accordingto the first embodiment.

As described above, also in the semiconductor device 200 according tothe present embodiment, it is possible to suppress increases of circuitscale and power consumption while keeping the frequency deviation withina predetermined allowable range.

<Control Method of Semiconductor Device>

Next, a control method of the semiconductor device according to thesecond embodiment will be described with reference to FIG. 11. FIG. 11is a flowchart showing the control method of the semiconductor deviceaccording to the second embodiment. In the description of FIG. 11, FIGS.8 and 9 are appropriately referred to.

As shown in FIG. 11, first, the reception detection unit RDU shown inFIG. 9 detects a frequency shift between the frequency signal fs and themaster frequency (step ST21).

Next, the switch control unit SC determines whether or not the frequencyshift of the frequency signal fs acquired from the reception detectionunit RDU is included in a predetermined allowable range (step ST22).

When the frequency shift of the frequency signal fs is included in thepredetermined allowable range (step ST22 YES), the switch control unitSC does not change the number of switches (the number of pairs ofswitches) to be ON and ends switch control without change. On the otherhand, when the frequency shift of the frequency signal fs is notincluded in the predetermined allowable range (step ST22 NO), the switchcontrol unit SC changes the number of switches (the number of pairs ofswitches) to be ON according to a value of the frequency shift of thefrequency signal fs and then ends the switch control.

Also in the semiconductor device 200 according to the presentembodiment, to reduce power consumption, the power source is turned offand the oscillation circuit OC stops every time packettransmission/reception is completed. Therefore, for example, every timea packet is transmitted/received, the switch control unit SC repeats thecontrol shown in FIG. 11 after the power source is turned on untilpacket transmission/reception is completed. The control shown in FIG. 11may be repeated every time a plurality of times of packettransmission/reception are performed, instead of every time a packet istransmitted/received. At a timing when the number of switches to be ONis changed, load capacity elements whose potentials are different arecoupled, so that the frequency of the frequency signal fs outputted fromthe oscillation circuit OC may largely vary. Therefore, it is preferablethat the timing when the number of switches to be ON is changed is otherthan during transmission/reception of a packet.

While the invention made by the inventors has been specificallydescribed based on the embodiments, it is needless to say that thepresent invention is not limited to the embodiments described above andmay be variously modified without departing from the scope of theinvention.

What is claimed is:
 1. A semiconductor device comprising: a crystal resonator; an oscillation circuit that oscillates the crystal resonator and outputs a frequency signal; a variable load capacity circuit including 1) a plurality of load capacity elements coupled in parallel with each other and coupled to one end of the crystal resonator and 2) a plurality of switches that are respectively serially coupled to the load capacity elements; a temperature sensor that detects a temperature of the crystal resonator and generates temperature information based on the detected temperature of the crystal resonator; and a switch control unit that controls ON/OFF of the switches based on the temperature information, wherein, when the temperature information that is used as an index of frequency deviation of the frequency signal due to a change in the temperature of the crystal resonator falls outside a predetermined allowable range, the switch control unit changes a number of switches to be turned ON among the plurality of switches to allow an absolute value of the frequency deviation to be small.
 2. The semiconductor device according to claim 1, wherein the information is frequency shift information between a frequency of the frequency signal and a master frequency of a wireless signal received from an external master wireless device.
 3. The semiconductor device according to claim 1, wherein the switch control unit changes the number of switches that will be turned ON among the plurality of switches during a period where no packet is transmitted or received.
 4. The semiconductor device according to claim 1, wherein capacities of the load capacity elements are equal to each other.
 5. The semiconductor device according to claim 1, wherein the variable load capacity circuit further includes a plurality of load capacity elements coupled in parallel to the other end of the crystal resonator and a plurality of switches that are respectively serially coupled to the load capacity elements.
 6. The semiconductor device according to claim 5, wherein a number of the load capacity elements coupled to one end of the crystal resonator and the number of the load capacity elements coupled to the other end of the crystal resonator are the same.
 7. The semiconductor device according to claim 5, wherein capacities of the load capacity elements coupled to one end and the other end of the crystal resonator are equal to each other.
 8. A control method of a semiconductor device including a crystal resonator, an oscillation circuit that oscillates the crystal resonator and outputs a frequency signal, a variable load capacity circuit including 1) a plurality of load capacity elements coupled in parallel with each other and coupled to one end of the crystal resonator and 2) a plurality of switches that are respectively serially coupled to the load capacity elements, and a temperature sensor that detects a temperature of the crystal resonator and generates temperature information based on the detected temperature of the crystal resonator, the control method comprising: acquiring the temperature information from the temperature sensor, and changing, when the temperature information that is used as an index of frequency deviation of the frequency signal due to a change in the temperature of the crystal resonator falls outside a predetermined allowable range, a number of switches to be turned ON among the plurality of switches to allow an absolute value of the frequency deviation to be small.
 9. The control method of a semiconductor device according to claim 8, wherein the information is frequency shift information between a frequency of the frequency signal and a master frequency of a wireless signal received from an external master wireless device.
 10. The control method of a semiconductor device according to claim 8, wherein the number of switches that will be turned ON among the plurality of switches is changed during a period where no packet is transmitted or received.
 11. The control method of a semiconductor device according to claim 8, wherein capacities of the load capacity elements are equal to each other.
 12. The control method of a semiconductor device according to claim 8, wherein the variable load capacity circuit further includes a plurality of load capacity elements coupled in parallel to the other end of the crystal resonator and a plurality of switches that are respectively serially coupled to the load capacity elements.
 13. The control method of a semiconductor device according to claim 12, wherein a number of the load capacity elements coupled to one end of the crystal resonator and the number of the load capacity elements coupled to the other end of the crystal resonator are the same.
 14. The control method of a semiconductor device according to claim 12, wherein capacities of the load capacity elements coupled to one end and the other end of the crystal resonator are equal to each other.
 15. A semiconductor device comprising: a crystal resonator; an oscillation circuit that oscillates the crystal resonator and outputs a frequency signal; a radio transceiver circuit that receives the frequency signal from the oscillation circuit and generates a transmission signal using the received frequency signal, the radio transceiver circuit wirelessly transmitting the generated transmission signal via an antenna; a variable load capacity circuit including 1) a plurality of load capacity elements coupled in parallel with each other and coupled to one end of the crystal resonator and 2) a plurality of switches that are respectively serially coupled to the load capacity elements; a temperature sensor that detects a temperature of the crystal resonator and generates temperature information based on the detected temperature of the crystal resonator; and a switch control unit that controls ON/OFF of the switches based on the temperature information, wherein, when the temperature information that is used as an index of frequency deviation of the frequency signal due to a change in the temperature of the crystal resonator falls outside a predetermined allowable range, the switch control unit changes a number of switches to be turned ON among the plurality of switches to allow an absolute value of the frequency deviation to be small. 